ITO is a degenerate semiconductor of n type whose transparency in particular in the visible range is due to the large band gap of more than 3.5 eV. The electric conductivity of ITO results from the supply of charge carriers such as electrons on levels close to the bottom of the conduction band firstly through the creation of oxygen vacancies and secondly by substitution, in the crystalline lattice, of indium ions by tin ions.
Therefore thin ITO layers are electrically conductive and optically transparent in the visible range. Through these characteristics, they have found a broad field of applications as electrode material in all technologies requiring transparent and conductive electrodes. Among the applications in which ITO layers can be used, mention may be made of optically transparent sensors or detectors [1], bioelectronic sensors or detectors [2], the micro/nano-structuring of optical devices but also screen technology for touch screens, solar cells, liquid crystal displays, light-emitting diodes, organic light-emitting diodes, heating devices for glass panes, mirrors or lenses etc. . . .
These applications mostly require the etching of layers (or films) of ITO with the selective removal of part of these layers. For this purpose, it is possible to use either wet etching or dry etching processes.
Wet etching is widely used since it offers a rapid, low-cost process. This technique is based on photolithography to achieve removal of the surface of the substrate. It can be applied to large surfaces with good reproducibility. However, there are a certain number of disadvantages such as problems of selective etching at grain boundaries and isotropic etching i.e. etching produced in all spatial directions. Small variations in the layer generate ITO residues after etching and also cause lateral attack of the ITO layers. This may deteriorate the electrical properties of the ITO electrode and lead to non-controllable forms with respect to the edges of the etched ITO layer. In addition, wet etching requires the use of strong acid solutions such as halogenated acids or aqua regia which may damage the film or substrate on which the ITO layer was deposited [3]. Wet etching also requires the use of a mask to protect the regions of the ITO layer which are not to be removed. The use of a mask not only makes the process more cumbersome with the need for additional steps such as the preparation, placing in position and removal of the mask, but also increases the cost of the process.
Dry etching particularly uses the deposit of the ITO layer by sputtering over the mask, and serigraphy. Another simple and efficient dry etching method is laser etching which does not require the use of masks. The laser photoenergy causes vaporization of the ITO layer and hence clean patterning of the ITO layer. However, even if the ITO layer can be completely removed with a single laser pulse, the ITO vapour may redeposit, after etching, on the etched surface [4]. Also, the energy of the laser may damage the substrate (or support) on which the ITO layer was deposited.
Finally, whether wet or dry etching is used, the final resolution obtained is of the order of 1 μm. In other words, these methods allow etching (or relief features) of the ITO layer to be obtained having a minimum width of 1 μm.
Having regard to the current potential of ITO layers and the applications thereof, there is a true need for a method with which it is possible to etch ITO layers in reproducible, precise manner without damaging the substrate on which the ITO layer was deposited.